Airport and en route air traffic control from ground installations relies upon the accuracy, resolution, and capacity of air traffic control systems. Typical air traffic control systems receive complex video data from rotating radar antennas and beacon interrogator sets. The signals from the surveillance system comprise radar echo signals as well as pulse trains from airborne transponders interrogated by the scanning ground beacon. A target extractor receives both sets of information as video data and converts these data into digital format for acceptance by a digital computer complex. The digital data include aircraft range (derived from radar echoes), azimuth (derived from the antenna position), and altitude, identity code (C or 3/A) and control flags (derived from the beacon transponder signal). The digital computer complex in turn processes and transmits data to the controller displays in the form of aircraft position signals. On each rotational scan of the radar/beacon set, hundreds of aircraft targets, some with overlapping transponder responses may be uncovered along with the usual clutter and random noise. This highly complex signal requires equally complex electronic circuitry. As critical decisions are made in reliance on the information displayed based on the video signals received from the actual targets, the air traffic control system must be tested regularly to insure its reliability.
Radar and beacon target report inputs, however, are not easily fabricated. The standard practice has been to use live radar targets (targets of opportunity), videotaped live radar targets (for repetitive testing), simulated static (not moving) radar and beacon targets, digital simulators capable of simulating the output of the radar preprocessor, such as the airport radial track system IIIA (ARTS IIIA) digital target simulator, and simulated inputs from the system's training target generator.
These past methods of providing radar and beacon targets have a number of disadvantages which can result in a defective or substandard air traffic control system receiving a passing grade. Live radar targets cannot be controlled; they are rarely found in large enough quantities to provide heavy system loading, and their tracks cannot be repeated.
While videotaping live targets affords repetition, the targets lack controllability and the taping process itself introduces serious degradation of the radar returns. Simulated static targets are only sufficient for testing the radar preprocessor; they do not allow the surveillance tracking system to be tested. On the other hand, digital target simulators, although able to test the tracking and display functions of the surveillance system, effectively bypass the radar preprocessor thus neglecting an important part of the system. Finally, the use of the training target generator bypasses the entire radar preprocessor and its associated input functions on the main processor. In addition, since the training target generator software operates in conjunction with the surveillance software and therefore interacts with the entire system, the training software's effect on the total system is not always readily detectable.
The foregoing techniques although they have many shortcomings, have been developed precisely because there has never been a suitable radar/beacon video traffice generator. The most critical functions of any air traffic control system involve the handling of closely spaced aircraft. Representing the video signals associated with with several moving intersecting clusters of dense air traffic requires electronic circuitry which has eluded the art up until now. Yet, this would be the ideal way of testing air traffic control systems at their "Achilles' heel" where mistakes are not only far more likely to occur but are also far more likely to have serious consequences.